1. Field of the Invention
The present invention generally relates to rolling door control apparatus, and more specifically to a failsafe automatic braking mechanism for a rolling door.
2. Description of the Prior Art
Numerous types of rolling doors have long be used particularly in commercial and industrial buildings. The general constructions and operations of such rolling doors are well known to those skilled in the art. Basically such rolling doors consist of a curtain of articulated slats that can be raised or lowered to selectively open and close a doorway or other opening in the building. The curtain is rolled and unrolled from a roll cylinder which is typically mounted horizontally across the top of the doorway. The roll cylinder is typically coupled to a drive for rotating the cylinder about its axis by means of a driven wheel coupled to the roll cylinder, a drive wheel mounted on a support shaft of a drive motor and a continuous loop drive which couples the drive and driven wheels. Most frequently the drive and driven wheels are chain drive sprocket wheels and the loop drive is a roller chain.
A major hazard that rolling doors present, however, is that potentially they can cause injury to operating personnel and/or damage by uncontrollably dropping due to failure of the system. Rolling doors can be quite heavy and although are typically provided with compensating systems to counterbalance the weight thereof such counterbalancing systems can and do fail. Such failures may result for many reasons including the braking of a counterbalance spring, a broken roller chain or power failure or reduction of voltage to the electric motor. When any of the aforementioned or other possible problems occur the door may be released in such a fashion that it undergoes free fail. As such rolling doors are frequently constructed of relatively heavy materials there is the possibility of extensive damage to the door slats or other slats or other system components and of course damage to property or injury to personnel located in or proximate to the door opening at the time of such free fall.
Due to the possibility of component or power failure, and to the fact that resultant free fall consequences have been known in the industry for some time, numerous efforts have been made to develop different types of stop mechanisms for rolling doors in the event of such anticipated failures. In some instances the stop mechanisms tend to be complex and, to that extent, tend to be less reliable since the stop mechanisms themselves can fail.
For example in U.S. Pat. No. 4,848,522 a safety device to prevent accidental unwinding of a rolling shutter or rolling overhead door is disclosed. The safety device includes a cog wheel which has uniformly distributed on the circumference opening radially movable tumblers. A cage surrounds the cog wheel that contains an abutment in the form of a recess for the tumblers that are raised by the action of centrifugal force. Upon reaching a predetermined rotational speed the tumblers engage an abutment in the cage and the cage, acting as a brake, is then driven by the cog wheel and slowed down by friction. However, such safety device is expensive and not extremely reliable. Also, by the nature of the mechanism the inertia of the components results in a time delay before a predetermined rotational speed is achieved. As the device is inherently slow considerable damage and/or injury may result before the desired braking action is achieved. Additionally the device has been known to jam at times at slow speeds which can cause damage to the drive motor.
In U.S. Pat. No. 4,112,996 a safety device is disclosed for arresting unrolling of a roller blind which is in some respects similar to the aforementioned patent in which an inertial member supported relatively movably with respect to a block shaft requires an increased unrolling velocity to cause the inertial member to change its relative movement and activate a gate member to release a pawl to permit it to engage the teeth of the gear and arrest unrolling. See also U.S. Pat. No. 5,494,093 which disclosed a rolling door stop apparatus in which a stop dog mounted on a pivot for rotation thereon is responsive to centrifugal forces created by rotation of rotor.
In U.S. Pat. No. 4,704,914 a chain breakdown safety device for a power-driven roller door is disclosed. In this device a steering rod is arranged on both sides of a chain between the sprocket and the chain wheel. A elongate movable rod is disposed typically perpendicularly to a line extending between the centers of the sprockets and the chain wheel respectively, the movable rod having a pair of slots formed therein for receiving the steering rod and a toothed rack portion. An idler roller is mounted on the movable rod for engaging a portion of the chain. Springs apply bias to the movable rod and the roller against the chain and a ratchet arrangement is provided for engaging with a toothed rack portion of the rod for causing a first engaging tooth to engage the toothed chain wheel when the movable rod shifts. In this manner the unrolling of the roller door is intended to be prevented should the chain fail. This arrangement is relatively complex and relies on a plurality of cooperating components to arrest the falling of the roller door. The stresses due to the momentum of force of rotation of the chain wheel can be significant, according to the disclosure, that a second engaging tooth needs to be used almost instantaneously to arrest the motion of the chain wheel.
In U.S. Pat. No. 3,704,757 an automatically braking mechanism is disclosed which includes a pivotally mounted brake arm having sprocket teeth engaging means movable into and out of engagement with the sprocket teeth of each chain drive sprocket wheel by means of a toggle linkage which when collapsed by the an air cylinder that moves the brake arm out of engagement and, when moved slightly past a dead center position by the air cylinder, moves the brake arm into engagement under the assistance of a torsion spring. This device does not directly monitor the integrity of the roller chain. Instead an air cylinder is relied upon to create the necessary air pressure, presumably once the door enters a free fall condition. Again, such an arrangement is unsatisfactory as it requires air pressure to build up and this may reflect an unacceptable delay before the toggle linkage actuates the brake arm.